This invention relates to ionization flow detectors for measuring the velocity of flow of a gas stream as well as for measuring the mass flow of said gas stream. Depending on whether the measurement is more especially oriented towards the mass flow rate or towards the gas flow velocity, these instruments have different names such as fluxmeters, anemometers, flow detectors or finally flowmeters.
The flow detectors of the prior art were initially constituted by a moving element which rotated under the influence of the gas stream. The speed of rotation of said element gave a measurement of the velocity of flow but the inertia of these instruments and the considerable changes in the flow regime caused by the presence of these latter are disadvantages which often prohibit their use. Furthermore, these mechanical devices are liable to lose their effectiveness in the course of time and can be employed only within limited ranges of temperature and pressure; they can also be employed only with non-corrosive gases.
Ionization flow detectors such as those described in Pat. Nos. EN 70 28636 corresponding to U.S. Pat. No. 3,750,469 and EN 71 18280 (corresponding to U.S. Pat. No. 3,835,705) relate to ionization flowmeters in which the ion source is produced by an electrode located in the gas stream. These ions are accelerated by means of another electrode placed opposite to the ion formation electrode. The ion path between the two electrodes is approximately perpendicular to the direction of the gas stream. The result achieved by this arrangement of electrodes is that, when the gas flows between said electrodes, the ions are entrained by the gas stream and are localized at a collection point on the electrode (or electrodes) which is no longer opposite to the formation electrode. A number of different systems have been proposed for measuring this displacement.
These systems in which the displacement of ions takes place at right angles to the direction of flow give rise to a number of disadvantages; in the first place, the sensitivity of such systems is fairly small since the measurement is applied to one velocity component at right angles to the primary ion velocity. Furthermore, the ions produced are of different types: the heavy ions arising from the presence of steam which condenses on the side walls of the flowmeter, namely the walls on which the electrodes are placed, have an ion velocity which is lower than that of the ions of the gas stream, thus causing increased deflection of the ion beam at a constant velocity of the gas stream. When heavy ions are present, the detectors which make use of this type of device provide an indication which is higher than the true value of the velocity of the gas stream and fail to ensure accuracy of response. Furthermore, the indications given by these detectors are extremely sensitive to variations in pressure of the gas which flows between the two electrodes as well as to temperature variations; the laws which give the rate of flow as a function of displacement, of temperature and of pressure are fairly complex laws which make corrections both uncertain and difficult.